This invention relates generally to a process for the introduction of additives into a melt extruder. More particularly it relates to the addition of color concentrates into a carrier polymer being processed in a melt extruder.
Thermoplastic polymers are often colored in the molten state by the addition of color concentrates. Color concentrates are concentrated dispersions of a colorant in a carrier polymer, the carrier polymer being either the same as or compatible with the polymer to be colored. There are several known methods for adding a color concentrate to a thermoplastic polymer.
One method is to meter a pelletized color concentrate into the throat of a polymer extruder using either volumetric or gravimetric pellet feeders. The volumetric system lacks sufficient precision for good color uniformity of the resulting polymer. Gravimetric feeders, while an improvement over volumetric feeders for color uniformity, are mechanically complicated and thereby both costly and difficult to maintain precise control.
Another known method of adding color concentrates to a thermoplastic polymer is to first melt the concentrate and then meter the molten concentrate into a melt of the thermoplastic polymer. This method provides for precise metering of the concentrate into the thermoplastic polymer thereby resulting in good color uniformity. However, the cost of melt addition equipment is high and achieving satisfactory levels of dispersion of color concentrate is sometimes difficult.
It is also known from GB No. 1,119,199 to meter color concentrates in a strand form to a rotary cutter located adjacent to the throat of an extruder. The rotary cutter forms pellets from the color concentrate which enter the extruder to mix with a thermoplastic polymer. This method aids somewhat in precise metering of pellets into an extruder; however, it still involves a modification of the color concentrate before it is added to the extruder.
In the known methods for adding color concentrates to thermoplastic polymers, it is always necessary to modify the color concentrate either by heating it to a molten state or by cutting it into pellet form.